This invention relates to a rotatable fluidised bed incinerator for incineration of combustible material.
Towns and cities are faced with an increasingly difficult problem in dealing with the disposal of wastes such as sewage. Traditionally, sewage sludge has been disposed of by open dumping, landfill or dumping in the sea. These practices either require large land acreage and are therefore becoming less attractive as the areas available for disposal near metropolitan areas rapidly disappear, or, in the case of sea dumping, are now or are shortly to be prohibited by new legislation.
Incineration of sewage sludge is recognised as an alternative possibility. Lurgi fluidised bed incinerators are widely used at present to burn sewage sludge. In this process the sludge must be de-watered to about 30% solids before combustion can be maintained. At this concentration of water, the sludge has the consistency of a thick paste and combustion takes place with no significant release of heat. If a filter press is used for de-watering to 35% solids, then the sludge has the consistency of cardboard and a significant heat output can be achieved. Lurgi fluidised bed incinerators are used to burn sewage sludge at a typical consumption rate of about 0.02 kg(dry)/s/m.sup.2 and, accordingly over 300 Lurgi style incinerators of 3 meters bed diameter would be required to cope with the expected future UK disposal requirements of about 5 million tonnes for the year.
Another possible approach is to dry the sludge to a dry powder and feed it into pf coal fired power station boilers. To feed paste sludge at, say, 30% solids into power station boilers would, however, require massive transport of an undesirable material which is difficult to handle.
A further problem with rotatable fluidised bed incinerators is the elutriation of matter either before or after combustion. This results in either the undesirable distribution of ash or less effective or incomplete combustion of waste material. Furthermore, the bed depth is typically relatively shallow which imposes limits upon the volume of material which can be processed by a fluidised bed incinerator at any given instant in time.